With a wider bandwidth of recent wireless communication, as a method capable of efficiently performing communication also in an ever-changing multipath propagation environment in which a plurality of incoming waves are present, there is widely used an OFDM (Orthogonal Frequency Division Multiplexing) for arranging signals in orthogonal frequencies (hereinafter, referred to as a subcarrier) and performing communication using a discrete Fourier transformation as one orthogonal transformation.
At the time of using the OFDM, a fast Fourier transformation (hereinafter, referred to as an FFT) as a fast algorithm of a discrete Fourier transformation and an inverse fast Fourier transformation (hereinafter, referred to as an IFFT) as an inverse transformation thereof are used in general. FIG. 2 is a conceptual diagram illustrating a transformation from a frequency domain to a time domain or from a time domain to a frequency domain of an OFDM symbol using the IFFT and FFT.
The transmitting station first allocates transmission signals to several subcarriers in the frequency domain and generates an OFDM symbol 301 in the frequency domain. The transmitting station substitutes zero into a subcarrier to which a signal is not allocated, and generates an OFDM symbol 302 with a length Td in the time domain using the IFFT. The transmitting station copies the OFDM symbol 304 with a length Tg as a part of the OFDM symbol 302 in the time domain and adds the copied portion to the head of the OFDM symbol 304. This added portion is hereinafter referred to as a guard interval. In addition, the same portion may be referred to a cyclic prefix, and is the same as the guard interval of the present specification.
The receiving station takes out only the OFDM symbol length Td from a reception signal and takes out a signal 308 in the frequency domain using an FFT operation. At this time, a range as a target of the FFT operation is hereinafter referred to as an FFT window. When a position of the FFT window with a length Td is matched with the OFDM symbol in the time domain excluding the guard interval, for example, as in a reference numeral 304, a signal 308 in the frequency domain after the FFT at the time of eliminating an influence of a propagation path is matched with the signal 301 in the frequency domain before the IFFT in the transmission side.
As compared with the above-described case, when the FFT window with a length Td is covered with the guard interval by a length Tlag, for example, as in a reference numeral 305, a phase rotation proportional to the amount of displacement Tlag in the time domain is applied to the signal 308 in the frequency domain after the FFT in addition to an influence of the propagation path. In addition, even if the phase rotation proportional to the amount of displacement Tlag is applied to the signal 308, when the phase rotation is compensated using a reference signal embedded within the OFDM symbol, a signal can be received similarly to a case where the phase rotation is prevented from being applied to the signal 308.
On the other hand, when a position of the FFT window protrudes out of a range of Tg+Td as in reference numerals 306 and 307, since interference from an adjacent OFDM symbol is applied to the FFT operation, degradation is generated in a quality of the reception signal. For this purpose, in the OFDM, a timing control of the FFT window position or transmission and reception of signals is required such that the FFT window falls in the range of Tg+Td. In an uplink of a multiuser system such as an SC-FDMA (Single Carrier-Frequency Division Multiple Access) and an OFDMA (Orthogonal Frequency Division Multiple Access) for receiving signals from a plurality of transmitting stations via one receiving station, the timing adjustment processing is particularly performed as disclosed in, for example, JP-A-2001-257641 “uplink timing synchronization and access control for a multi-access wireless communication system” such that the transmission time of each transmitting station is corrected by an instruction from the receiving station and signals from each transmitting station fall in the range of the FFT window of the receiving station.
FIG. 3 is a conceptual diagram illustrating the timing adjustment processing. In the timing adjustment processing, the FFT window of the receiving station is determined and a signal timing 310 for receiving signals as a reference at a timing of this FFT window is determined. At this time, a reference numeral 311 denotes an example of the reception signal at a reference reception timing. In the timing adjustment processing, when the reception signal is received at a timing advanced from the reference timing, for example, as in a reference numeral 312, the receiving station instructs the transmitting station to transmit signals so as to delay the reception timing by the amount of displacement 313 between the reference reception timing and the reception timing of a reference numeral 312. Also when the reception signal is received at a timing delayed from the reference timing, for example, as in a reference numeral 314, the receiving station similarly instructs the transmitting station to transmit signals so as to advance the reception timing by the amount of displacement 315 between the reference reception timing and the reception timing of a reference numeral 314.    Patent Document 1: JP-A-2001-257641